(1) Field of the Invention
The present invention relates to a process for the slurry polymerization of formaldehyde. More particularly, it relates to a process for the slurry polymerization of formaldehyde in which an undesirable deposition or build up of polyoxymethylene onto the inner wall of a polymerization reactor and a stirrer is minimized.
(2) Description of the Prior Art
A most serious disadvantage encountered in carrying out the slurry polymerization of formaldehyde in an inert organic liquid medium is that formed polyoxymethylene is deposited onto the inner wall of a polymerization reactor and onto a stirrer and, consequently, continuation of stirring and effective removal of the heat of polymerization become difficult, and it becomes necessary to stop the polymerization reaction frequently and wash the polymerization reactor and stirrer, with the result that it is impossible to conduct the continuous operation stably.
Some proposals have heretofore been made in order to eliminate the above-mentioned disadvantages caused by the deposition of polyoxymethylene. For example, it has been proposed in Japanese Patent Publication No. 10,548/1971 to employ a polymerization vessel fitted with a stirring chain which is rotated to impinge against the inner wall of the polymerization vessel, thereby to peel off the polyoxymethylene deposited thereon. It also has been proposed in Japanese Patent Publication No. 3,261/1972 to blow an inert gas into the polymerization mixture during polymerization to prevent polyoxymethylene from being deposited onto the inner wall of the polymerization vessel or the stirrer.
The above-mentioned proposals are not satisfactory. That is, in the former proposal, it is impossible to uniformly peel off polyoxymethylene adhering to the inner wall of the polymerization vessel, and since the peeled blocks of polyoxymethylene are incorporated in the polymer slurry withdrawn from the polymerization vessel, after-treatments such as the terminal group-stabilizing treatment and the copolymerization are adversely affected by the presence of such peeled blocks of polyoxymethylene. When the latter method is carried out on an industrial scale, a special apparatus must be disposed and used for circulating the inert gas. Furthermore, the yield of the polymer is reduced.
It now has been found that the undesirable deposition of polymer is caused mainly by the fact that polymerization of formaldehyde is induced partially by a free catalyst present on the surface of the inner wall of a reactor and on the surface of a stirrer. Accordingly, in the polymerization of formaldehyde using a homogeneous catalyst, it is difficult to prevent such deposition of polymer.
It also has been proposed in Japanese Laid-open Patent Application No. 35,791/1978 to polymerize formaldehyde in an inert organic liquid medium in the presence of a polymerization catalyst adsorbed on polyoxymethylene. This catalyst adsorbed on polyoxymethylene is prepared by a process wherein a polymerization catalyst such as diethylzinc, tin tetrachloride and boron trifluoride ethyl etherate is brought into contact with polyoxymethylene in an inert organic liquid medium whereby the polymerization catalyst is adsorbed on the polyoxymethylene, and then, the catalyst-adsorbed polyoxymethylene is filtered off from the liquid medium and then heat-treated so that adhesion of the catalyst to the polyoxymethylene is enhanced. However, the above-proposed method is still not satisfactory in that, even when the catalyst is contacted with polyoxymethylene over a long period of time, only a minor quantity of the catalyst can be adsorbed on polyoxymethylene.